Method and apparatus for average current control

ABSTRACT

A method and apparatus for controlling a converter circuit within the electrical accumulator unit based on a comparison between an actual average converter current and a desired average converter current.

BACKGROUND OF THE INVENTION

The present application is directed toward a control system for anelectrical accumulator unit.

In order to provide power to electrical systems, many vehicles, such asmilitary aircraft, feature an on-board generator that convertsrotational movement within the engines to electrical power using knownpower generation techniques. The generated electrical power is used topower on-board electrical components such as flight controls, sensors,or weapons controls. During standard operations, such a system will havean electrical load that normally draws power at a certain level. If someon-board electrical systems, such as weapons systems, are activated atemporary elevated load spike can occur.

In order to compensate for the temporary load spike, a generator istypically used that is rated at least as high as the highest anticipatedpower spike. This ensures that adequate power can be provided to theon-board electrical systems at all times, including during elevated loadspikes. One device, which is used to manage the elevated load spikes isan electrical accumulator unit. The electrical accumulator unit can acteither as a power source or as a power sink, and thereby allows for asmaller generator to be used.

SUMMARY

Disclosed is a method for controlling a power converter. The methodincludes the steps of determining an operating mode, determining adesired average converter current, determining an actual averageconverter current over a time period equal to one switching cycle of thepower converter, comparing the desired average converter current withthe actual average converter current, and outputting a control signalcapable of adjusting a converter current based on said comparison.

Also disclosed is an electrical accumulator unit that has a power filterwith electrical connections for connecting to a power bus, a powerconverter connected to the power filter, a power storage componentconnected to the power converter, and a controller. The controller iscapable of controlling the power filter, the power converter, and thepower storage component based on a desired average current and an actualaverage current.

These and other features of the present invention can be best understoodfrom the following specification and drawings, the following of which isa brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a sample aircraft having an on-board power generationsystem.

FIG. 2 schematically illustrates an aircraft power generation systemincluding an electrical accumulator unit.

FIG. 3A schematically illustrates an electrical accumulator unit.

FIG. 3B schematically illustrates a buck-boost power converter circuitwhich could be used in the example of FIG. 3A.

FIG. 4 illustrates an example control loop for controlling a buck-boostconverter operating in a boost mode.

FIG. 5 illustrates an example control loop for controlling a buck-boostconverter operating in a buck mode.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates a sample aircraft 10 having an on-boardpower generation system. A generator 20 converts rotational motionwithin an engine 22 into electrical power using known power generationtechniques. The generator 20 is electrically coupled to a rectifier 30.The rectifier 30 converts the power generated in the generator 20(typically three-phase power) into a form usable by on-board electronics50 (typically DC power). The rectifier 30 is electrically coupled to apower bus 40 that supplies power to the on-board electronics 50 or thelike through power supply lines 42. Additionally connected to the powerbus 40, is an electrical accumulator unit 60, which can store excesspower generated by the generator 20 if the load needed by the on-boardelectrical system 50 is low, and reinsert that power into the powersystem when the load needed by the on-board electrical system 50undergoes a high load spike.

FIG. 2 schematically illustrates a power generation system 100 describedwith regards to FIG. 1. A three phase generator 110 is connected to anAC/DC rectifier 120 via three phase outputs 112A, 112B, 112C. The threephase generator 110 may also be referred to as generator 110. The AC/DCrectifier 120 converts the generated three phase power into DC power,and outputs the DC power to a power bus 130. Connected to the DC powerbus 130 is a variable load 140. The variable load 140 (also referred toas an external load) may represent a variable number and size ofelectrical loads that can change over time and/or be selectively added,removed, or modified. Additionally connected to the DC power bus 130 isan electrical accumulator unit 150 (EAU). The three phase generator 110,AC/DC rectifier 120, DC power bus 130, variable load 140, and electricalaccumulator unit 150 represent embodiments of the generator 20,rectifier 30, power bus 40, the load created by the on-board electronics50, and electrical accumulator unit 60 of FIG. 1 respectively. Agenerator controller 160 (also referred to as controller 160) isconnected to both the electrical accumulator unit 150 and the threephase generator 110, and provides control signals for both. Thegenerator controller 160 is also connected to the output of the AC/DCrectifier 120 via power sensors, and is capable of detecting the poweroutput of the AC/DC rectifier 120 and the power demands of the variableload 140. Alternately, the electrical accumulator unit 150 can becontrolled by an independent controller.

The example power generation system 100 of FIG. 2 generates power at itsmaximum rating and the variable load 140 uses less than all of thegenerated power under normal conditions. The excess power is absorbed bythe electrical accumulator unit 150, which stores the excess power in apower storage component such as a battery or ultra capacitor or acombination of both. When the variable load 140 spikes, and exceeds thegenerating capacity of the generator 110 the electrical accumulator unit150 reverses and begins supplementing the power provided to the DC powerbus 130 with the power which has been stored within the power storagecomponent, thereby ensuring that the variable load 140 receives adequatepower throughout the high power spike. When absorbing power, theelectrical accumulator unit 150 is referred to as operating in a buckmode, and when supplementing/delivering power, the electricalaccumulator unit 150 is referred to as operating in a boost mode.

FIG. 3A illustrates a schematic diagram of an example electricalaccumulator unit 200. The electrical accumulator unit 200 and power bus250 represent embodiments of the electrical accumulator unit 150 and DCpower bus 130 of FIG. 2. The electrical accumulator unit 200 has threeprimary components, an energy storage unit 220, a power converter 230,and a filter 240. Additionally included in the example of FIG. 3A is anelectrical accumulator unit controller 260. Use of a separate controller260 to control the electrical accumulator unit 200 can allow forlocalized control circuits. Alternatively, the electrical accumulatorunit 200 could be controlled by controls housed within the generatorcontroller 160, as in the example of FIG. 2. The filter 240 is acombination of a ripple filter and an electromagnetic interference (EMI)filter. The ripple filter portion of the filter 240 removes ripplecurrents that have leaked onto the power bus 250 due to the presence ofpower electronics in the load such as variable load 140 of FIG. 2, ordue to the action of the power converter 230. Similarly, the EMI filterportion of the filter 240 filters out electromagnetic interferencepresent on the power bus 250. Ripple currents and electromagneticinterference are common occurrences in such electrical systems andresult from the connection of the power bus 250 has to the variable loadas well as the electrical systems exposure to other sources ofelectrical noise. Allowing the interference and ripple currents to reachthe power converter 230 is undesirable.

After passing through the filter 240, the electrical power enters abi-directional power converter 230 where it is converted from the formof dc electrical power used by the power bus 250 into a form which canbe accepted and stored by the energy storage component 220. Thebi-directional power converter 230 is also capable of converting poweroutput from the energy storage component 220 into the form used on thepower bus 250 if the electrical accumulator unit 200 is providing powerto the system, such as during a high load spike or while operating inemergency mode. Furthermore, by controlling the current passing throughthe converter 230, a controller can control the rate at which the energystorage component 220 is charged and discharged, and thereby control theelectrical accumulator unit 200 functions. A method for controlling thecurrent passing through the converter 230 is described below.

The energy storage component 220 can be any device or component that iscapable of accepting power from the power converter 230 and storing thatdc power for later use. In the illustrated example of FIG. 3A, a batteryor ultra capacitor (ultra cap) or a combination of both could be used.However, other energy storage components could be used with minormodifications to the electrical accumulator unit 200.

FIG. 3B illustrates a more detailed example embodiment of a powerconverter 230, such as would be used in FIG. 3A. The power converter 230has a high side power connector 350 and a low side power connector 352,which connect the power converter 230 to the filter 240 (illustrated inFIG. 3A). The example circuit illustrated in FIG. 3B is a bi-directionalbuck-boost converter that uses an inductor filter 362, a boost switchthat has a transistor 370 and a diode 364, and a buck switch that has atransistor 368 and a diode 366 to reduce the voltage from the power bus250 to a level acceptable by the energy storage component 220 whilepower is being stored, and to raise the voltage level of the power beingproduced by the energy storage component 220 when the energy storagecomponent 220 is providing power to the power bus 250. It isadditionally possible to include standard sensors (not pictured) capableof detecting the current passing through the inductor 362, the buckswitch transistor 368, or the boost switch transistor 370.

While the example buck-boost converter uses a single buck-boost circuit,a functionally similar circuit could be used which includes severaliterations of the illustrated buck-boost circuit connected in paralleland phase shifted to work as a single unit. The inductor filter 362,diodes 364, 366 and transistors 368, 370 need not be implemented asdiscrete components. The transistors 368, 370 in the buck-boost circuitcan be controlled by the generator controller 160 (illustrated in FIG.2) according to known control techniques, by a central power systemcontroller which can control other load and storage units in modernaircraft electrical power systems, or by an independent electricalaccumulator unit controller.

In order to control the power supplying/siphoning functions of theelectrical accumulator unit 200 during normal operations, the controller260 can place the electrical accumulator unit 200 in a boost mode(supplying power to the DC power bus), a buck mode (receiving power fromthe DC power bus), or a stand-by mode where the electrical accumulatorunit 200 performs neither function. The electrical accumulator unit 200defaults to operating in the stand-by mode. For example, if it isdesirable to siphon power from a power bus, the controller 260 cangenerate a control signal which switches the electrical accumulator unit200 from the stand-by mode to the buck mode.

The controller 260 also includes an average current sub-controllercomponent for controlling the current flowing through the converter 230in the electrical accumulator unit 200. This sub-controller componentallows the controller 260 to control the converter 230 current based ona measured average current through the converter 230, as will bediscussed below.

FIG. 4 schematically illustrates a feedback control circuit 500 (alsoreferred to as a boost mode control loop 500) of the controller 260 forcontrolling the converter 230 in a boost mode. The system illustrated inFIG. 4 includes three control system inputs, a desired average currentinput 522, a measured inductor current input 524, and a boost-mode PWMclock signal input 526 into a flip-flop logic circuitry 510. A clocksignal input is a pulse signal that occurs at a set frequency and can begenerated by a standard controller using known techniques. The frequencyof the clock signal input 526 is set by the controller 260, and is theswitching frequency of the converter 230. The clock signal input 526starts a Pulse Width Modulation (PWM) cycle of the power converter 230in the boost-mode. The PWM frequency of the power converter 230 canrange from kilohertz to megahertz depending on the capability of thepower switching converter 230, and is decided by the designers duringthe design of the power system. The cyclic operation of the powerconverter 230 in boost mode begins when the first clock signal input 526is received by the flip-flop logic circuitry 510.

The desired average current input 522 receives a command signalrepresenting a desired average inductor current from the controller 260,which allows the converter 230 to optimally boost the power to the DCpower bus 250. This desired current is determined by the controller 260using known techniques. The desired current signal 522 is subtracted bya slope compensation signal 528, generated by the controller 260, in asubtraction block 530. The slope compensation signal 528 is a downwardsloping signal that is periodic and shares a period with the clocksignal input 526, and the PWM switching frequency. The inductor currentinput 524 receives a sensor output corresponding to the inductor currentof the inductor 362 in the converter 230. The inductor 362 current canbe measured using any known technique. The slope compensation signal 528is determined by the power converter designer during the design of thesystem and is configured to stabilize the power converter 230, while itis operated in boost-mode. By way of example, the slope compensationsignal 528 could be a signal that starts at a high value, and decreasesto a low value at a consistent pace over the course of a singleiteration of the power converter's power cycle. Such a signal isreferred to as a down ramp signal.

The boost mode control loop 500 integrates the received inductor current524 using an integrator 540 that is reset at the beginning of each PWMcycle when a falling edge detector 560 outputs a reset signal. Thefalling edge detector 560 detects when the integration cycle is endingby detecting a falling edge of the control signal 512 (i.e. a flip-floplogic circuitry 510 output signal 512 changes from “1” to “0”). Thefinal value of the output of the integrator before being reset is theaverage inductor current of the PWM cycle. The average current duringthe boost mode is determined using the formula:

$I_{average} = {\left( \frac{1}{T_{s}} \right){\int_{0}^{Ts}{I_{Inductor}\ {{t}.}}}}$

Ts is the period of the clock signal 526. The determined average valueis sent from the output of the integrator 540 to an input of thecomparator 550.

The comparator 550 compares the average inductor current value from theintegrator 540 to the desired average inductor current value from thecompensated command signal output from the subtraction block 530. Whenthe inputs of the comparator 550 equal each other, the comparator 550outputs a signal to reset the flip-flop logic circuitry 510 to “0”, andto reset the integrator 540. While the output of the flip-flop logiccircuitry 510 is “1”, the boost switch 370 (of FIG. 3B) is turned-on,and while the flip-flop logic circuitry 510 is “0” the boost switch 370is turned off, thereby controlling the converter. The control output 512is also linked to a falling edge detector 560.

In theoretical control systems, level changes of the control signal areperformed instantaneously. In practical systems, if a control signalswitches from a high signal to a low signal there is a lag time duringwhich the voltage rapidly declines, rather than an instantaneousdecline. This rapid decline is referred to as a falling edge. Similarly,transitioning from a low signal to a high signal causes a rising edge inpractical systems. When a falling edge is detected, the falling edgedetector 560 emits a pulse, which resets the integrator 540 to a “0”value, and starts the next iteration of the boost mode control loop 500.

Thus, the boost mode control loop 500 causes the controller 260 toaccurately adjust the converter 230 current based on a measured averageconverter current 524, and thereby allows for precision control of theconverter current in a boost mode. In boost mode, the integrator 540continuously integrates the incoming converter currents 524.

A similar feedback loop is illustrated in FIG. 5, which schematicallydepicts a feedback control loop 600 (also referred to as a buck modecontrol loop 600) for a converter 230 operating in buck mode. The buckmode control loop 600 is similar to the boost mode control loop 500,with like numerals indicating like elements. The buck mode control loop600 operates similar to the boost mode control loop 500 illustrated inFIG. 4 and described above. The buck mode control loop 600 differs fromthe boost mode control loop 500 in three ways. In buck mode, themeasured current 624 for the integrator 640 input is the current througha buck switch 368 rather than the current through the inductor 362 as inthe boost mode.

The second difference between the two modes is the operation of theintegrator 640. As described above with regard to FIG. 4, the integrator540 continuously integrates and is reset to “0” each iteration of theboost mode control loop 500. Control signals for operating in a buckmode are non-continuous and therefore cannot be continuously integrated.A non-continuous signal includes breaks where no control signal isoutput. During those breaks, the integrator 640 would not function.Consequently, the integrator 640 is started and stopped for eachiteration of the buck mode control loop 600 rather than run continuouslyas in the boost mode control loop 500. To facilitate starting andstopping the integrator 640 during each iteration, a rising edgedetector 670 detects a rising edge of the control signal at the start ofeach iteration, and starts the integrator 640. When the iteration of thefeedback loop ends, there is a falling edge on the control signal. Whenthe falling edge detector 660 detects the falling edge, the integrator640 is stopped, the average current is output to the comparator 650, andthe integrator 640 is reset. The formula used to determine the averagecurrent in buck mode is

$I_{average} = {\left( \frac{1}{T_{s}} \right){\int_{0}^{Ts}{I_{buckswitch}\ {{t}.}}}}$

The third difference is which converter switch 368, 370 is beingcontrolled. In the buck mode the control loop 600 controls the buckswitch 368. As with the boost mode, when the flip-flop logic circuitry610 is “1,” the buck switch 368 is on, and when the flip-flop logiccircuitry 610 is “0,” the buck switch 368 is off.

The controller 260 can be configured such that each of the buck controlloop 500 and the boost control loop 600 use shared components, where thecomponents are redundant, or configured with two independent controlpaths that are switched between based on a desired operating mode signal526, 626.

While the above converter circuit controller has been described withrelation to an electrical accumulator unit, it is understood that acontrol circuit according to the present disclosure could be utilizedwith a PWM converter control circuit, and is not limited to electricalaccumulator unit applications. Although an example embodiment has beendisclosed, a worker of ordinary skill in this art would recognize thatcertain modifications would come within the scope of this invention. Forthat reason, the following claims should be studied to determine thetrue scope and content of this invention.

What is claimed is:
 1. A method for controlling a power convertercomprising the steps of: determining an operating mode using acontroller; controlling a power converter by performing the steps of:determining a desired average current based at least partially on saidoperating mode using the controller; determining an actual averagecurrent over a time period equal to one switching cycle of the powerconverter; comparing said desired average current flow with said actualaverage current; and outputting a control signal capable of adjusting acurrent based at least partially on said comparison.
 2. The method ofclaim 1, wherein said step of determining an operating mode furthercomprises selecting an operating mode from a list comprising a boostmode, a stand-by mode, or a buck mode.
 3. The method of claim 2, whereinsaid step of determining an actual average current comprises integratinga measured inductor current over a time period equal to one switchingcycle of the power converter, and dividing the integrated inductorcurrent by a value equal to a number of seconds in said time period,thereby determining an average current when said operating mode is aboost mode.
 4. The method of claim 3, wherein the measured inductorcurrent is an inductor filter current.
 5. The method of claim 3, whereinsaid step of determining an actual average current further comprises thesteps of detecting a falling edge of a control signal and resetting anintegration value to a starting value when said falling edge is detectedand said operating mode is a boost mode.
 6. The method of claim 2,wherein said step of determining an actual average current furthercomprises the steps of integrating a measured transistor current over atime period equal to one switching cycle of the power converter anddividing the integrated transistor current by a value equal to a numberof seconds in said time period, thereby determining an average currentwhen said operating mode is a buck mode.
 7. The method of claim 6,wherein said measured transistor current is a buck switch transistorcurrent.
 8. The method of claim 6, wherein said step of determining anactual average current when said operating mode is a buck mode furthercomprises the steps of: detecting a rising edge of a control signal;starting an integrator in response to said rising edge; detecting afalling edge of a control signal; stopping an integrator in response tosaid falling edge; integrating a measured current over a time periodstarting at said rising edge and ending at said falling edge; dividingsaid integrated measured current by a number of seconds in said timeperiod, thereby determining an average converter current.
 9. The methodof claim 8, further comprising the steps of resetting an integrator toan original value when a falling edge is detected.
 10. The method ofclaim 2, wherein said steps of determining a desired average currentusing the controller, determining an actual average current over a timeperiod equal to one switching cycle of the power converter, comparingsaid desired average current with said actual average current, andoutputting a control signal capable of adjusting a current based atleast partially on said comparison are suspended while said operatingmode is a stand-by mode.
 11. The method of claim 1, wherein said step ofdetermining a desired average current comprises the steps of: receivinga desired average current value at a summer from a controller; receivinga stabilizing signal at said summer from the controller; summing saiddesired average current value and said stabilizing signal using saidsummer; and outputting a stabilized desired average current value fromsaid summer.
 12. The method of claim 11, wherein said stabilizing signalcomprises a down ramp signal.
 13. An electrical accumulator unitcomprising: a power filter having electrical connections for connectingto a power bus; a power converter connected to said filter; a powerstorage component connected to said power converter; and a controllercapable of controlling said power filter, said power converter, and saidpower storage component based on a desired average current and an actualaverage current.
 14. The electrical accumulator unit of claim 13,wherein said controller comprises a buck mode control loop circuit and aboost mode control loop circuit controllably connected to at least oneof said power converter, said power filter, and said power storagecomponent, each of said control loop circuits having inputs forreceiving a desired average current value from a controller.
 15. Theelectrical accumulator unit of claim 13, wherein said power converterfurther comprises a filter inductor, a boost switch, and a buck switch.16. The electrical accumulator unit of claim 15, wherein said powerconverter further comprises a sensor capable of detecting a currentpassing through said filter inductor.
 17. The electrical accumulatorunit of claim 15, wherein said power converter further comprises asensor capable of detecting a current passing through said buck switch.18. A method for controlling an electrical accumulator unit currentcomprising the steps of: determining an operating mode using acontroller; controlling an electrical accumulator unit power converter,and thereby controlling said electrical accumulator unit poweraccumulation functions by performing the steps of: determining a desiredaverage converter current based at least partially on said operatingmode using the controller; determining an actual average convertercurrent over a time period equal to one iteration of the method;comparing said desired average converter current with said actualaverage converter current; and outputting a control signal capable ofadjusting a converter current based at least partially on saidcomparison.